This invention relates to a strain detector and, more particularly, to a strain detector for detecting a strain on a driven shaft such as a rotary shaft generated when an external force is applied.
FIG. 13 illustrates an example of a conventional strain detector in which reference numeral 1 designates a driven shaft, and 2 designates its central axis. Reference numerals 3 and 4 designate bearings for the driven shaft 1, 5 and 6 are magnetic layers attached to an outer circumferential surface of the driven shaft 1 at angles of +45.degree. and -45.degree. and made of a highly permeable, soft magnetic material having a predetermined suitable magnetostrictive constant, 7 designates a coil bobbin supported by the bearings 3 and 4 around the driven shaft 1, 8 and 9 are detection coils wound around the coil bobbin 7 corresponding to the magnetic layers 5 and 6, respectively, 10 and 11 are magnetic yokes made of a highly magnetically permeable, soft magnetic material such as an amorphous alloy or a silicon steel sheet and disposed on an outer circumference of the detection coils 8 and 9, and 12 designates a non-magnetic shield made of a non-magnetic, highly electrically conducive material such as Cu and Al and disposed around the yokes 10 and 11 in common. The reference numeral 13 designates a magnetic shield disposed around the non-magnetic shield 12. The magnetic shield 13 is made of a highly magnetically permeable, soft magnetic material such as an amorphous alloy or a silicon steel sheet. A detection circuit 14 is connected to the detection coils 8 and 9.
When an external torque is applied to the driven shaft 1, a tensile stress is generated in one of the magnetic layers 5 and 6 and a compression stress is generated in the other of the magnetic layers 5 and 6, whereupon a strain is generated in each layer. This strain causes a re-arrangement of magnetic domains in the magnetic layers as a result of magnetostriction phenomenon, which in turn causes a change in magnetic permeability of the magnetic layers. At this time, the permeability varies in opposite directions depending upon the tensile stress or the compression stress. The detection coils 8 and 9 detect this change in magnetic permeability as a change in magnetic impedance and supply their outputs to the detection circuit 14, where a detection voltage V indicative of a magnitude of the torque applied on the driven shaft 1 is provided.
The magnetic yokes 10 and 11 are for concentrating the magnetic fluxes generated from the detection coils 8 and 9 in the magnetic layers 5 and 6 to flow therethrough and to prevent the leakage of the magnetic flux, thereby to increase the sensitivity. The non-magnetic shield 12 is made of a non-magnetic, high electrical conductivity material, so that the alternating magnetic flux appears only in a very shallow skin region of the material, whereby a magnetic flux generated by the detection coil and an external a.c. magnetic flux are magnetically isolated by the non-magnetic shield 12. Therefore, the leakage of the magnetic flux generated by the coil is prevented, whereby the sensitivity is increased, and the undesirable ingress of the external a.c. magnetic flux is prevented so that the noise immunity is improved. Also, the magnetic shield 13 is made of a highly magnetically permeable, soft magnetic material, so that the ingress of an external d.c. magnetic field to the magnetic layers 5 and 6 is prevented.
In the above arrangement, the yokes 10 and 11 which are made of a high permeability soft magnetic material such as an amorphous alloy or a silicon steel sheet are disposed around the detection coils 8 and 9 for concentrating the magnetic flux generated from the detection coils 8 and 9 to flow through the magnetic layers 5 and 6, thereby to preventing the flux leakage and increasing the sensitivity.
However, the magnetic yokes are simple tubular members, so that the function of concentrating the magnetic flux in the magnetic layers 5 and 6 is not sufficient for completely eliminating the leakage of the magnetic flux to the bearings 3 and 4, whereby the sensitivity of the strain detector is degraded. Also, some portion of the magnetic flux generated from the detection coils 8 and 9 reaches to the other detection coil. As a result, errors in the detection torque are increased.